(1) Field of the Invention
This invention relates to refrigerant flow rate control devices, and more particularly it is concerned with a refrigerant flow rate control device suitable for use with the refrigeration cycle of an air conditioning system for an automotive vehicle.
(2) Description of the Prior Art
One type of the refrigeration cycle of an air conditioning system for an automotive vehicle is disclosed in U.S. Pat. No. 4,129,995, in which a temperature sensor and a pressure sensor are located at the outlet of an evaporator to enable a refrigerant to evaporate optimally in the evaporator in accordance with an external heat load. The temperature sensor and pressure sensor monitor the temperature and pressure, respectively, of the refrigerant and produce signals, and sensing of superheat is effected based on these signals. Valve opening regulating means is provided to regulate the degree of opening of a refrigerant flow rate control valve in such a manner that the superheat sensing coincides with a predetermined value.
In the valve opening regulating means, various types of actuator may be used to cause the degree of opening of the refrigerant flow rate control valve to change. They may include the internal pressure of a gas-sealed cylindrical temperature sensitive member, a solenoid device and a stepping motor. However, in an air conditioning system for an automotive vehicle, a negative pressure in the suction manifold of an engine may be advantageously used when weight, motive-power that is required and reliability in performance are taken into consideration.
In a refrigerant flow rate control device of the prior art driven by a vacuum serving as an actuator, a sealing member, such as a bellows, which is capable of freely changing its shape is used to provide a cover to a portion of a valve body extending through a valve drum to effectively prevent leaks of the refrigerant to the atmosphere through the interface between sliding surfaces of the valve body and valve drum and enable displacements of the valve body to take place without any trouble. This makes the pressure applied by the refrigerant to the sealed portion covered with the sealing member equal in level to a pressure at the down-stream end portion of an orifice, so that a force is exerted by the difference between this value and the atmospheric pressure to move the valve body to an open position.
Generally, the refrigeration cycle has the risk that when the compressor is inoperative, a liquid refrigerant might flow into the compressor and compression of the liquid refrigerant occurs as the compressor is started again, thereby causing damage to the compressor. To avoid this trouble, it is necessary that the refrigerant flow rate control valve be completely shut off when the compressor is rendered inoperative. To this end, it has hitherto been usual practice to use a spring of high resilience as a restoring spring, so that the biasing force of the restoring spring will overcome the force exerted by the pressure of the refrigerant acting on the sealed portion to shut off the valve when the compressor is rendered inoperative. Meanwhile, when the valve is opened, it is necessary to drive the valve body by overcoming the biasing force of the restoring spring. This has given rise to the problem that the size of a diaphragm should be increased sufficiently to increase a drive force exerted by the vacuum actuator.
On the other hand, U.S. Pat. No. 3,684,238 discloses an electromagnetically controlled fluid pressure modulating valve including a valve body formed with a through hole so as to equalize pressures that are applied to opposite ends of the valve body. However, this valve is a solenoid valve and an existing vacuum actuator is unusable as it is with this valve. When the existing vacuum actuator is used, a drive force produced by the vacuum actuator should be mechanically transmitted to the valve to drive the valve body. Thus, the valve body should have a portion extending through a valve drum and exposed to the atmosphere. This makes it necessary to provide seal means to the portion of the valve body extending through the valve drum to prevent leaks of a refrigerant from the refrigeration cycle to the atmosphere while allowing the valve body to move through the valve drum. Thus, it would be impossible to apply the valve shown in U.S. Pat. No. 3,684,238 to a refrigerant flow rate control device.